This invention relates generally to a valve for controlling the unidirectional flow of a pulsatory fluid. More particularly, this invention relates to a new and improved valve prosthesis which provides for the nontraumatic control of body fluid flow through the use of active means which consists of power generated by electromagnetic energy. This invention is particularly well suited for use as a valve prosthesis for controlling blood flow through the heart of a human or animal.
Artificial valves, both those used as prosthetic devices within the human and animal body and those used to control the flow of body fluids externally, have long been known to the medical profession. The prior art valves used primarily as prosthetic devices have taken many forms in an attempt to replicate the function of the natural valves which they replace. None of these prior art valves, however, has fully achieved the replication of a naturally occurring valve. Lack of success has resulted in part from problems with long term durability of the valve itself and from differences between actual and measured fluid flow area provided by the valve which has resulted in undesirable pressure gradients across the valve. In addition, prior art valves increase rather than reduce or eliminate, the turbulence created by the passage of fluid through and the subsequent closure of the valve. Such turbulence results in greatly increased hemolysis when the fluid flowing through the valve is blood.
Prior art prosthetic valves conform, for the most part, to three general types including ball and cage valves, tissue PG,3 valves and disc type valves. The ball and cage valve, such as that described in U.S. Pat. No. 3,416,159, is usually constructed of silicone rubber, is susceptible to variance in shape, fractures of the ball surface and subsequent lipid infiltration into the ball. Additional complications which often accompany this type of valve include fibrin clot formation, which further interfers with the movement of the ball in the cage, and thrombosis. Because of these complications, the patient with this type of prosthesis must undergo continuous anticoagulant therapy. Moreover, since the ball portion of the valve must be located in the center of the cage to allow proper functioning of the valve, fluid flow through this type of valve cannot be central, as in a natural valve, but is directed through the valve around the periphery of the ball. The turbulence resulting from this, coupled with the turbulence created by closure of the valve, causes a higher level of hemolysis and blood cell damage that is undesirable. The presence of the ball in the center of the blood vessel also imparts an unnatural radial component to the flow of blood within the vessel producing injury to the walls of the vessel against which the radial component is directed. Ball and cage valves are large in size and therefore can present difficulties in insertion. They have also been disturbing to some persons into whom they have been inserted because of the audible sound detectable upon closing. Attempts to correct the deficiencies of the ball and cage type valve by replacing the ball with a disc shaped element have generally been unsuccessful.
A second type of prior art valve, generally referred to as a tissue valve, is composed of a stent or mounting ring to which human or animal tissue has been attached in a form which approximates the flaps in a natural valve. However, the long-term durability of these valves is still of concern. Construction of the tissue valve from cadaver material such as fascia lata results in a valve with low durability because of rapid tissue degradation and stiffening and has led to tissue dysfunction and subsequent valve immobility. Moreover, since these valves rely entirely on back pressure for closure, closure is accompanied by increased trauma to the blood cells and the greater likelihood of valve incompetence than encountered with a natural valve.
A third category of prosthetic valve is the disc type, which is generally formed of an annular base to which a disc-shaped valving member is secured, either by means of a magnetic hinge, as in U.S. Pat. No. 3,370,305 to Goot, by means of an eccentrically-placed stem, as in the Modified University of Capetown Prosthesis described by Ellis et al, in The Annals of Thoracic Surgery 23:26-31, 1977, or by means of a rod along which the disc can be displaced, as in U.S. Pat. No. 3,959,827 to Kaster. Although fluid flow through this type of valve is more centralized than through the ball and cage valve, increased hemolysis results from turbulence created by fluid flow through the valve. In addition, injury to the walls of the blood vessel beyond the valve results from the radial components imparted to the blood flow caused by the presence of the valve disc in the blood stream. Moreover, use of this valve necessitates the institution of long-term anticoagulant therapy because of the high level of thrombogenesis which accompanies the use of the valve. As reported in The Annals of Thoracic Surgery 23:26-31, 1977, Ellis et al discontinued use of one type of tilting disc prosthesis, in part because of the excessive incidence of thromboembolism and in part because the effective valve orifice area, after the valve had been in place, was usually considerably less than the measured orifice area before insertion.
The tilting disc is designed to pivot on an off center axis. To move from its closed position to its open position, the disc rotates on its axis to some position less than 90 degrees from its closed position. The valve structure has a mechanical stop to inhibit the disc from rotating to a position completely parallel to the direction of flow. Therefore when reverse flow occurs, the disc will be inclined to rotate back to its closed position because the force created by the reverse flowing fluid will be greater on one side of the disc than on the other. While repositioning the disc from its open position to its closed position, a finite amount of fluid (termed ""regurgitation") is allowed to pass through the valve in the reverse direction because the valve is not able to close instantaneously. Paradoxically, the quantity of reverse flow is minimized by reducing the pivot rotational angle of the disc and the quantity of laminar-forward flow is maximized by increasing the pivot rotation angle of the disc parallel to the direction of flow. In other words, one can increase laminar flow in the forward direction at the cost of regurgitation of the reverse direction.
In U.S. Pat. No. 3,959,827, Kaster discloses one embodiment of a disc type valve in which the closing of the valve is assisted by means of a permanent magnet located in the disc valving member. Although this permits a smoother closing than is possible without the magnet, the disc valve disclosed in U.S. Pat. No. 3,959,827 still suffers from the other drawbacks generally common to disc type valves. Forman et al reported in The Journal of Thoracic and Cardiovascular Surgery 75:595-598, 1978, that they no longer recommended the use of a tilting disc valve similar to the one disclosed in U.S. Pat. No. 3,959,827, in large part because of a high incidence of embolism and valvular thrombosis, but also because the effective orifice area of the valve was less than the actual orifice area. In addition, they found this type of valve demonstrated no clear hemodynamic advantage over other available prosthesis.
Valves used for medical purposes in which valve closing is magnetically assisted are also disclosed in U.S. Pat. Nos. 3,233,610 to Wade, 3,495,620 to Raimondi et al and 3,926,175 to Allen et al. The valves described in U.S. Pat. Nos. 3,233,610 and 3,495,620 provide fluid flow orifices through only a portion of the valve diameter, thus resulting in a slow fluid flow area in relation to valve diameter. The magnetically actuated valve disclosed in U.S. Pat. No. 3,926,175 is incapable of operating in response to fluid pressure within a body vessel and is thus unable to operate automatically to cause unidirectional flow of a pulsatory fluid. Magnetic repulsive forces have been used to aid valve operation as illustrated in U.S. Pat. No. 3,476,355 to Sherwood but has never been used to prevent traumatic valve operation which could result in injury to body fluids.
Other more recent magnetically assisted artificial heart valves are disclosed in U.S. Pat. No. 4,417,360 to Moasser and U S. Pat. No. 4,605,408 to Carpentier. Moasser discloses a prosthetic valve comprised of an annular mounting structure to which is attached at their proximal ends two opposed, spaced flexible flaps. These flaps include at their distal ends permanent magnetic members with sufficient magnetic force to prevent traumatic closure and to reduce the systolic pressure required upon opening. Carpentier relates to a prosthetic heart valve of the disc type which uses two small permanent magnets to enhance opening of the valve with the two magnets exerting a controlled repellant effect on one another. Carpentier also states that the permanent magnetics may be electromagnetic, but there is no disclosure of how this would be accomplished.